1. Field of the Invention
This invention relates to an improved seal system designed for particular use in completion and production operations of oil and gas wells wherein the seal system greatly reduces the tendency and incidence of seal seizure or sticking.
2. Description of the Prior Art
Movement of tubing within the sealing bores of packers, bridge plugs, and the like, during completion and production operations in high pressure, high temperature oil and gas wells has been a major cause of damage to seal systems and subsequent diminished performance and sealing integrity of such seal systems. Damage to the seal systems has become a greater problem during the present intensive search for new oil and gas reserves, wherein the drilling and subsequent completion is in deeper wells, involving exposure in extremely hostile high temperature, high pressure environments wherein the well production may contain a significant amount of hydrogen sulfide, carbon dioxide and methane.
To overcome these conditions and thus successfully complete such a well, the seal system must have continuous sealing integrity. The seal system must be resistant not only to the damaging effects of the well environment (i.e., temperature, pressure, fluids) but also to the physical stresses imposed by or resulting from completion or workover techniques, i.e., unrestrained tubing movement, build up of corrosive products in the sealing annular area, and the like. Typical of prior art seal systems is U.S. Pat. No. 2,862,563, illustrating a well packer assembly for packing the annular space between tubing in a well, wherein resilient annular packing elements are spaced about a tubular mandrel. U.S. Pat. No. 3,083,775 teaches the use of a formation packer wherein a plurality of resilient annular packing elements are spaced about a tubular mandrel and a plurality of folded metal plates which are set on a double traveling mandrel. U.S. Pat. No. 3,531,236 discloses a tubular sealing assembly having chevron sealing rings formed from a fluoroelastomer and asbestos, with fluorocarbon plastic ring adaptors at each end of the seal stack. U.S. Pat. No. 2,467,822 teaches the use of a rubber or similar packing material which is prevented from flowing through the opening between the packer body and the packing retainer or abutment surrounding the body.
The prior art also shows a number of generic seal systems which may have utility in the sealing of a well conduit. For example, U.S. Pat. No. 3,467,394 shows a packing means of a V-ring type wherein the packing arrangement comprises a polytetrafluoroethylene, commonly marketed under the trademark TEFLON, with relatively rigid V-ring shaped spacer rings interposed between a plurality of the elastomeric V-rings. Additionally, U.S. Pat. No. 4,050,701 shows ring seals obtained from a mixture of polyphenylene sulfide and polytetrafluoroethylene, for use in the fluid sealing of rotary and/or reciprocating shafts. Similarly, U.S. Pat. No. 3,626,337 discloses a packing ring for use in high temperatures and pressure environments wherein a thermoplastic type composition, such as rubberized Nylon, tetrafluoroethylene, polyesters, acrylics, and the like, are laminated to form the final seal substance. U.S. Pat. No. 3,799,454, discloses a coating composition containing polytetrafluoroethylene and polyenylene sulfide for formation of a seal system.
Generally, the seal systems of the prior art have not been totally suitable for use in wells having high bottom hole temperature and pressures and corrosive fluids, and have also been found to be deficient when exposed to even less severe environments.
To overcome these deficiencies it has been suggested that various new elastomeric type materials be used in the sealing systems. A report of the 52nd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers, Oct. 9-12, 1977 discusses the testing of a number of the various new elastomeric type materials in relation to their use in packer seal systems. This Report No. SPE 6762 discloses the use of three particular elastomeric type materials which can be utilized in packer seal systems. These elastomeric materials are commonly sold under the trademarks of TEFLON, a polymer of tetrafluoroethylene, RYTON, a polymer of polyphenylene sulfide, and KALREZ, a perfluoroelastomer.
Polytetrafluoroethylene, sold under the DuPont trademark TEFLON, is a flexible fluoropolymer having a high degree of permanent set and cold flow. It also has a high resistance to corrosive chemicals and high temperatures. It can be used as virgin TEFLON or it may be filled with a suitable filler, i.e., glass particles.
Polyphenylene sulfide resin, sold under the trademark RYTON by Phillips Petroleum Company, is a thermoplastic resin characterized by a high thermal stability, excellent chemical resistance, and a high affinity for fillers. The resin may be the reaction product of p-dichlorobenzene and sodium sulfide in a polar solvent, as disclosed in U.S. Pat. No. 3,354,129.
The perflouroelastomer, sold under the trademark KALREZ by DuPont, is a material characterized by high thermal stability and excellent chemical resistance. Exemplary of such perfluoroelastomers is the reaction product of perfluoro(3-phenoxypropylvinyl ether) and at least one fluorine-containing ethylenically unsaturated monomer, as fully disclosed in U.S. Pat. No. 3,682,872.
The above described elastomeric materials have been used in packer seal systems, but not with complete success. It has been found that seal systems which incorporate these elastomeric materials have a strong tendency to adhere or stick to the conduit when the seal system must be retrieved from the well.
These problems were found to be particularly prevalent in a commercially available seal system incorporating elastomeric materials wherein the seal system comprised six seal assemblies with each seal assembly, in turn, comprising, in order, a KALREZ seal unit, a 25% glass-filled TEFLON seal unit, and a RYTON seal unit. Each seal assembly was separated by rigid metallic spacing members abutting a seal unit comprising either KALREZ or RYTON. For testing purposes, this seal system was incorporated into a packer wherein the rigid spacing member abutted the KALREZ seal unit. Pressure was thereafter applied to the seal system. The spacing member transmitted the applied pressure to the seal assembly and the seal assembly transmitted the pressure induced upon it to the other adjacent seal assemblies. It was found that the seal assemblies held satisfactorily under the transmitted pressure; however, the KALREZ seal unit "flowed" back over the abutting metal spacers. This backward flow not only destroyed the elastomeric seal unit, but it caused extremely high frictional forces which, in turn, caused sticking that would resist subsequent retrieval of the well packer and the seal system.
The sticking of the seal system can occur when there are multiple seal assemblies wherein the upper seal assembly holds, and thus the lower seal assembly receives only the mechanical load and not the hydraulic pressure of the seal system. The mechanical load will cause the elastomer, i.e., the KALREZ, to extrude in both directions, i.e., the direction from high to low pressure and also in the direction of high pressure itself. The metal spacer adjacent the elastomeric material acts like a hydraulic ram and the elastomer can extrude back over the spacer as the mechanical load acts upon it. This action causes the seal system to have a high friction-to-movement and it is this friction, as hereinbefore mentioned, which causes the seal system to stick, i.e., to resist subsequent retrieval. This obviously may cause severe damage to the seal system.
The present invention relates to an improved seal system which greatly reduces sticking and enables the packer, bridge plug or the like, together with the seal system to be more easily retrieved. The present invention is also designed to greatly reduce the seal damage associated with seal systems incorporating elastomeric materials in hostile physical environments in a well.
The reduction in sticking and seal damage is accomplished in accordance with this invention by providing a seal assembly which prevents the elastomer from extruding back over the adjacent rigid spacing member. The sealing system is characterized by its ability to withstand hostile environments in wells which have high pressures and temperatures, corrosive chemicals, etc. The system also withstands dynamic tube movement of the packer and minimizes sticking problems due to extrusion of elastomers of the seal members into the gap between the seal bore and spacing members.
It is an object of the invention to provide a seal system usable on a conduit as a permanent seal system in the completion of and production from, corrosive, high pressure, high temperature wells.
It is another object of the invention to provide a seal system which reduces seal damage caused by movement of the associated conduit in the bore of an oil or gas well during completion and production operations.
It is another object of the invention to provide a seal system which reduces the tendency of a seal system to stick to a conduit during retrieval of the seal system.
Other objects of the invention will be apparent from the following description.